The present invention relates to capacity control and, more particularly, to a capacity-controlling method and apparatus which are suitable for a compressor such as a multistage centrifugal compressor which has coolers between the stages thereof, or an axial-centrifugal compressor which has coolers between an axial compression stage and a centrifugal compression stage.
In, for example, a multistage centrifugal compressor, capacity control is performed by providing vanes at the entrances of individual stages and operating the vanes, with a typical system being proposed in U.S. Pat. No. 4,288,198. In this proposed control method, the vanes of each of a plurality of stages are moved experimentally to confirm changes in efficiency, so that the efficiency peak can be found for each.
However, with this method, the relationships of the vane openings of each of the stages are mechanically fixed in advance as linear relationships. In other words, the vane openings of the individual stages are determined independently of the flow rate of the compressor, so that the vane opening of a downstream stage is linearly determined once the vane opening of the first stage has been determined.
The individual stages of a multistage compressor have intrinsic, different fluid performances, so that its overall performance is determined by a combination of different performances. In a conventional linear combination of individual vane openings, e.g., in an air separation plant, the turn-down capacity is technically acceptable at about 70% of the rated capacity, and higher efficiencies are expected within that range. The recent energy-saving trend has led to a demand for a further enlargement of the control range and higher efficiencies when partially-loaded. In the prior art, however, as noted, above these demands cannot be satisfied because the relationships of the vane openings at individual stages are fixed.